John M. Olson / Professor of Photosynthesis (retired), Odense University

05 April 2011

In the fall of 1951, I arrived at the Johnson Foundation as a graduate student in biophysics. My first two years were devoted to taking courses, one of which was Plant Physiology taught by Dave Goddard. As I recall, it was Goddard who drew my attention to photosynthesis as a really interesting area for research. At about the same time Britton Chance (BC) and Lucile Smith were looking into cytochrome reactions in Rhodospirillum rubrum, a purple non-sulfur photosynthetic bacterium. Then Martin Kamen came along with great enthusiasm about a new purple bacterium Chromatium strain D, that used H2S as the H-donor for CO2 fixation in photosynthesis. He convinced me (and BC) that Chromatium would be an excellent choice to study the role of cytochromes in an organism that carried out photosynthesis without interference from photoassimilation.

By 1955, I had learned how to grow these bacteria, and I had built a cross-illuminator for the sample cell of a Chance double-beam spectrophotometer, which was also used by Martin Klingenberg to study rat-liver mitochondria. He used the instrument in the morning and afternoon, while I usually began work in the late afternoon and continued on into the night. BC was my standard for hours devoted to research in the lab, but I never exceeded his record.

From endless recordings of light-induced absorbance changes in suspensions of bacteria under aerobic and anaerobic conditions and in the presence of CO, I was able to detect the involvement of four cytochromes in the light reactions: C423.5, C426 (a CO-binding pigment), C422, and C430. (The various species were named after the positions of their Soret peaks in the reduced-minus-oxidized difference spectra. The most light-sensitive cytochrome was C423.5, and BC suggested that I determine the quantum requirement for oxidation of this cytochrome. One year later I had the result: 2 quanta per electron at both 11 and 29 C. We considered this to be good evidence that the oxidation of C423.5 was the result of a direct electron transfer from cytochrome to the primary oxidant created by the initial photochemistry of photosynthesis.